Alternator based battery charging systems use a voltage regulator to develop a charging voltage for maintaining a battery fully charged to a predetermined voltage level. Some systems require the charging voltage to have a negative temperature dependence. This requirement is caused by the need for a greater charge voltage at lower temperatures and a lesser charge voltage at higher temperature. Some of these systems require substantially no temperature variation of the charge voltage and then a negative temperature dependence of charge voltage above a predetermined threshold temperature in order to prevent overcharging the battery at high temperatures. Current systems do not accurately define this threshold temperature. Deficiencies include a soft transition around the threshold temperature that can vary by 20.degree. C. Current systems are also impractical because they use additional and costly components and manufacturing processes. These deficiencies are undesirable and can cause poor field reliability.
What is needed is an improved thermally dependent self-modifying voltage source useable in a voltage regulator of an alternator based battery charging system that overcomes the deficiencies of the prior art.